1. Field of the Invention
The present invention relates, in general, to a spacer grid used for placing and supporting a plurality of nuclear fuel rods within a nuclear fuel assembly and, more particularly, to a nuclear fuel spacer grid with dipper vanes designed to maintain a desired space between the elongated fuel rods and to appropriately support the fuel rods and to prevent the vibrating or fretting corrosion of the fuel rods while allowing coolant to smoothly flow within the fuel assembly and resulting in a high coolant mixing effect and retaining a desired structural strength of the fuel assembly even in case of the occurrence of an emergency.
2. Description of the Prior Art
In conventional light water reactors, a plurality of elongated nuclear fuel rods 125 are regularly and parallelly arranged in an assembly 101 having a square cross-section in a way such that, for example, fourteen, fifteen, sixteen or seventeen fuel rods 125 are regularly arranged along each side of the square cross-section, thus forming a 14xc3x9714, 15xc3x9715, 16xc3x9716, or 17xc3x9717 array as shown in FIG. 1. In such a nuclear fuel assembly 101, the elongated fuel rods 125, fabricated by containing a fissile material within a hermetically sealed elongated zircaloy tube 114 known as the cladding, are placed and supported by a plurality of spacer grids 110. Each of such spacer grids 110 is produced by welding a plurality of intersecting grid strips to each other into an egg-crate pattern prior to encircling the periphery of each grid 110 by four perimeter strips. The top and bottom of the fuel assembly 101 are, thereafter, covered with pallets 111 and 112, respectively. The fuel assembly 101 is thus protected from any external loads acting on the top and bottom thereof. The spacer grids 110 and the pallets 111 and 112 are also integrated into a single structure using a plurality of guide tubes 113. A framework of the fuel assembly 101 is thus fabricated.
Each of the above spacer grids 110 is fabricated as follows. As best seen in FIG. 2, two sets of grid strips 115 and 116, individually having a plurality of notches at regularly spaced positions, are assembled with each other by intersecting the two sets of strips 115 and 116 at the notches, thus forming a plurality of four-walled cells individually having four intersections 117. The assembled strips 115 and 116 are, thereafter, welded together at the intersections 117 prior to being encircled by the perimeter strips 118, thus forming a spacer grid 110 with such four-walled cells. As shown in FIG. 3, a plurality of positioning springs 119 and a plurality of positioning dimples 120 are integrally formed on or attached to the rid strips 115 and 116 in a way such that the springs 119 and the dimples 120 extend inwardly within each of the four-walled cells. In such a case, the dimples 120 are more rigid than the springs 119. In each of the four-walled cells, the positioning springs 119 force a fuel rod 125 against associated dimples 120, thus elastically positioning and supporting the fuel rod 125 at four points within each of the cells. In the above nuclear fuel assembly 101, a plurality of grids 110 are regularly and perpendicularly arranged along the axes of the fuel rods 125 at right angles, thus placing and supporting the fuel rods 125 at multiple points. The grids 110 thus function as a multi-point support means for placing and supporting the fuel rods 125 within the fuel assembly 101.
In conventional light water reactors of Korea, water is used as coolant. In such a light water reactor, water receives thermal energy from the fuel rods 125 prior to converting the thermal energy into electric energy. During an operation of the nuclear fuel assembly 101 of such a reactor, water or liquid coolant is primarily introduced into the assembly 101 through an opening formed on the core supporting lower plate of the reactor. In the above assembly 101, coolant flows upwardly through the passages, defined between the fuel rods 125, and receives thermal energy from the fuel rods 125. The sectioned configuration of the coolant passages, provided within the fuel assembly 101, is shown in FIG. 4.
Since the elongated, parallel fuel rods, having a circular cross-section, are closely arranged within a fuel assembly, having a rectangular configuration, while being spaced apart from each other at irregular intervals, the temperature of coolant flowing around the fuel rods is variable in accordance with positions. Therefore, it is almost impossible to obtain a uniform temperature distribution within such a conventional fuel assembly. Thus, the coolant passages of the fuel assembly may be partially overheated at positions adjacent to the fuel rods having a high temperature. Such partially overheated regions of the coolant passages deteriorate soundness of the assembly and reduce the output power of the fuel rods. In order to remove such partially overheated regions from a nuclear fuel assembly, it is necessary to design the spacer grids in a way such that the grids allow a uniform temperature distribution of the coolant within the fuel assembly while effectively deflecting and mixing the coolant within the assembly. A conventional example of such designed grids is disclosed in Korean Patent Publication No. 91-7921. In the grid disclosed in the above Korean patent, so-called xe2x80x9cmixing bladesxe2x80x9d or xe2x80x9cvanesxe2x80x9d are attached to the upper portion of each grid and are used for mixing coolant within the fuel assembly. That is, the mixing blades or vanes allow the coolant to flow laterally in addition to normally longitudinally, and so the coolants are effectively mixed with each other between the passages and between the lower temperature regions and the partially overheated regions of the fuel assembly.
The important factors necessary to consider while designing the grids 110 for use in nuclear fuel assemblies are improvement in both the fuel rod supporting function of the grids 110 and the buckling strength resisting a laterally directed force acting on the grids 110. During an operation of a nuclear reactor, the fuel assembly 101 may be vibrated laterally due to a load acting on the assembly and this causes an interference between the fuel rods within the assembly. Therefore, the grids of the fuel assembly may be impacted due to such an interference between the fuel rods as disclosed in U.S. Pat. No. 4,058,436. In the prior art, the grid""s buckling strength, resisting a lateral load acting on the grid, is reduced since the grid strips have to be partially cut away through, for example, a stamping process at a plurality of portions so as to form positioning springs 119 and dimples 120 within a fuel assembly. Such cut-away portions (or windows) reduce the effective cross-sectional area of the grid 110 capable of resisting impact, thus reducing the buckling strength of the grid 110. Therefore, it is almost impossible to avoid a reduction in the buckling strength of the grid 110 in a conventional design of grids wherein the grid strips have to be partially cut away to form such positioning springs 119 and the dimples 120.
In the conventional nuclear fuel assembly 101, the fuel rods 125 are placed and supported by positioning springs 119 and the positioning dimples 120 within the grids 110. However, the lateral flow or mixing of coolant regrettably vibrates the elongated, parallel, closely spaced fuel rods 125 within the assembly, and so the fuel rods 125 easily and periodically interfere with the intersecting strips of the grids 110. When the fuel rods 125 are so vibrated for a lengthy period of time, the claddings of the fuel rods 125 are repeatedly and frictionally abraded at their contact parts at which the fuel rods 125 are brought into contact with the springs and dimples of the grids 110. The claddings are thus reduced in their thicknesses so as to be finally perforated at the grid contact parts. Such an abrasion of the fuel rods 125 is so-called xe2x80x9cfretting wearxe2x80x9d in the art. It has been noted that such a fretting wear is caused when there is any gap between the fuel rods, the positioning springs and the positioning dimples. However, such a fretting wear may be prevented by a structurally improved spacer grid capable of more effectively placing and supporting the fuel rods within the fuel assembly. Therefore, it is necessary to provide a spacer grid having a fuel rod supporting structure designed to more firmly support the fuel rods in comparison with conventional grids having one spring and four dimples or two springs and four dimples.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a nuclear fuel spacer grid of which each grid strip has a lower dipper vane at its lower end so as to control the inflow direction of coolant and an upper dipper vane at its upper end so as to swirl the outflow coolant, thus allowing the coolant to more strongly swirl while flowing within a fuel assembly and improving the thermal efficiency of the fuel assembly.
Another object of the present invention is to provide a nuclear fuel spacer grid, of which the grid strips are not cut away (i.e. windowless) at any portion for forming separate spring or dimples, thereby having a desired mechanical and structural strength, such as a desired buckling strength capable of effectively resisting lateral load acting thereon, and which uses thin grid strips capable of having a desired strength expected from conventional thick strips, thus reducing resistance to the flow of coolant within the fuel assembly and improving the thermal hydraulic performance of the fuel assembly.
A further object of the present invention is to provide a nuclear fuel spacer grid, which increases the number of fuel rod supporting points and supports the fuel rods at its upper and lower ends, thus more effectively supporting the fuel rods while protecting the fuel rods from fretting wear.
The nuclear fuel spacer grid of this invention comes into contact with the fuel rods at positioning springs without having dimples, thus being effectively increased in flexibility in comparison with conventional grids having both the springs and the dimples.
In order to accomplish the above objects, the present invention provides a nuclear fuel spacer grid fabricated by intersecting a plurality of zircaloy or inconel grid strips and used for placing and supporting a plurality of elongated fuel rods within a nuclear fuel assembly. In the spacer grid of this invention, each of the grid strips is not cut away to form separate springs or dimples for supporting the fuel rods, but has an axial slot extending from one end of each strip to a length. The grid strips are intersected at the slots prior to being welded together into a single structure at a tap formed at an outside end or a middle portion of each of the slots. Each of the grid strips also has a dipper-shaped coolant mixing vane, or a dipper vane, at each of upper and lower ends thereof. The spacer grid thus maximizes the thermal hydraulic coolant mixing effect within a fuel assembly and stably supports the fuel rods within the assembly while improving the mechanical/structural strength of the assembly, such as a buckling strength.